Continuous tuning of persistent luminescence wavelength by intermediate-phase engineering in inorganic crystals

Multicolor tuning of persistent luminescence has been extensively studied by deliberately integrating various luminescent units, known as activators or chromophores, into certain host compounds. However, it remains a formidable challenge to fine-tune the persistent luminescence spectra either in organic materials, such as small molecules, polymers, metal-organic complexes and carbon dots, or in doped inorganic crystals. Herein, we present a strategy to delicately control the persistent luminescence wavelength by engineering sub-bandgap donor-acceptor states in a series of single-phase Ca(Sr)ZnOS crystals. The persistent luminescence emission peak can be quasi-linearly tuned across a broad wavelength range (500–630 nm) as a function of Sr/Ca ratio, achieving a precision down to ~5 nm. Theoretical calculations reveal that the persistent luminescence wavelength fine-tuning stems from constantly lowered donor levels accompanying the modified band structure by Sr alloying. Besides, our experimental results show that these crystals exhibit a high initial luminance of 5.36 cd m−2 at 5 sec after charging and a maximum persistent luminescence duration of 6 h. The superior, color-tunable persistent luminescence enables a rapid, programable patterning technique for high-throughput optical encryption.

One of the reasons for this study is mentioned in the introduction: PersL emitters of various emission colors usually exhibit unbalanced brightness and efficiency.Unfortunately, this is also the case in the present solution: changing the Ca-Sr ratio also changes the trapping behaviour (as witnessed in the TL glow curves and in the intensity of the decay curves (Fig. 3)).In that sense, the present phosphor system is not a real solution, where spectral tuning can be achieved independently of the trapping behaviour.
Of course, the study by itself contains interesting results, but I am not convinced that the proposed material system will have a high impact in the field, given that the aforementioned problem is not really solved.In the application part, a 'super broadband PersL' is created by mixing two Ca-Sr compositions.Doesn't this create the same issue, regarding color changes during the decay?Furthermore, in the abstract it is claimed that the high initial brightness is up to 5 cd/m², although I could not find data supporting this claim.All the data for the decay curves are shown in arbitrary units, except for supplementary figure 9, but there the initial intensity is lower than 5cd/m².Furthermore, it is mentioned that it was measured with a (scanning) spectrometer, calibrated with a luminance meter.More details should definitely be provided on the calibration method, as the luminance (measured in cd/m²) depends on the shape of the spectrum.Hence, a calibration of the FLS980 spectrometer to yield cd/m² values for widely different emission colors is not straightforward.The advise would be to measure the phosphors directly, after excitation, with the luminance meter, and then compare them.Preferably, also standard excitation conditions (Xe lamp and D65 lamp) should be used to give a fair estimate of the obtainable brightness.
It would also be interesting to plot in Fig. 3d the peak and fwhm not only for the PL, but also for the PersL, and discuss in more detail the differences.
There is an issue with the TL curves in Figure 4.The erratic way in which the plot lines are shown does not have a meaning (a TL measurement assumes a constant heating rate, now the curves jump back and forth as a function of temperature).Also, it is mentioned that the TL glow curves are nearly identical, but the differences are actually quite large and they do not change in a very systematic way when the composition is changed.For a publication in NatComm, this should be investigated in more detail.
I will not comment on the theoretical calculations, as I am not an expert in the field.Regarding the conclusion from the Se substitution, it is a bit surprising to read that the PersL is strongly correlated with sulfur, while it is actually a vacancy.This should be explained in more detail.
Finally, I must admit that -even after reading several times -I didn't get how the programmable optical storage would actually be used in practice.In panel f, other information appears compared to panel e (Figure 5), but how does this change evolve over time, and what is then the "useful" time period to do the read out?I do not see where an optical information storage during 5s (or 60s) would be useful for.Also for the multicolor display, there is some uncertainty about what is exactly done.Was the phosphor excited through the patterned film?Or would such a 'display' be excited without the pattern, after which the user inserts the film?Furthermore, is the emission spectra stable through the mentioned duration of 6 hours?Is it then still possible to see color information?

Response:
We are grateful for your valuable suggestion.The latest advancements in PersL spectrum tuning are concisely documented in Supplementary Table 1 in the revised Supplementary Information.The prevalent method for PersL color modulation entails the amalgamation of diverse emission centers, including triplet states, guest chromophores, and inorganic activators, into a singular host matrix.These strategies yield specific emission bands yet impede the comprehensive full-spectrum expression of PersL.Response: Thank you for raising this point.Donor-acceptor (D-A) luminescence originates from the recombination of an electron bound to a donor with a hole bound to an acceptor, both of which are defect levels within the semiconductor's bandgap.Our previous research revealed that Cu/RE (RE = Y, Gd, Tb, Nd, Er, Ho, Tm, Dy, and Pr) can form D-A pairs, leading to band emissions in addition to the characteristic luminescence from the RE ions in CaZnOS.

Supplementary
Consequently, in the current study, we have employed the non-luminescent Y as a co-activator to exclusively generate the D-A emission and modulate the persistent luminescence (PersL) spectrum.
The D-A emission's tunability was demonstrated by substituting Cu/Y with other D-A pairs.For instance, using Cu/Gd resulted in a continuously adjustable D-A spectrum, comparable to Cu/Y (Supplementary Fig. 12a-b).With Cu/Tb, the resulting spectrum was a composite of D-A and Tb ionic emissions, with the D-A luminescence being constantly adjusted with increasing Sr concentration (Supplementary Fig. 12c-d).Additionally, we observed emissions in Ag/In co-doped Ca(Sr)ZnOS, in which the emission peaks shifted to a shorter wavelength as the Sr content increased, despite without a detectable PersL signal (Supplementary Fig. 12e).
We have added the following discussion in the revised manuscript: "Concurrently, our prior investigations have demonstrated that, in CaZnOS, copper and rare-earth elements (e.g., Y, Gd, Tb, Nd, Er, Ho, Tm, Dy and Pr) are capable of forming D-A pairs.This interaction facilitates band emissions, which occur alongside the intrinsic lanthanide luminescence.

Reviewer #2 (Remarks to the Author):
This paper reports an interesting wavelength tuning method for persistent luminescence (PersL) by intermediate-phase engineering.Using this method, the PersL wavelength could be tailored over a wide range with a high precision.This is a big step forward in terms of wavelength diversification compared to existing material design routes.Although the manuscript is well organized and written in general, there are some points that are unclear and need further polishing.Considering the potential impact in the field optoelectronic materials, I would like to suggest the publication of this paper in Nat.Commun.after appropriate revisions.
Response: Thank you for your recognition and positive comments.
1.The authors claim that the emission centers in the Ca(Sr)ZnOS:Y/Cu compounds are donoraccepter (D-A) pairs, which majorly come from the results of first-principle calculations (DFT).
Firstly, it should be made clear whether they are donors/acceptors of electrons or holes.
Secondly, is it possible that the emission centers are attributed to Cu + , Cu 2+ or other Cu-related defect species, similar to the case of ZnS:Cu.

Response:
We apologize for any confusion.In semiconducting materials, donor impurities donate additional electrons, whereas acceptor impurities are responsible for generating free holes.Radiative recombination is initiated when the wavefunction of an electron confined within a donor site overlaps with that of a hole in an acceptor, leading to donor-acceptor (D-A) luminescence.To verify the D-A emission nature, we have checked the peak and FWHM for both PL and PersL (Fig. 3d).The peak wavelength of PL is shorter (corresponding to higher emission energy) than that of PersL for the series of CaZnOS:Cu/Y crystals and the FWHM values of PL are larger than the correspondent PersL.The difference can be understood by considering the transition energy of D-A pair luminescence, which can be expressed as: where Eg is the bandgap energy, ED/EA is the ionization energy of the donor/acceptor, " is the dielectric constant of the crystal and r is the distance between D-A pair.During photoexcitation, the average distance between donor-acceptor pairs significantly decreases.This reduction in distance results in an increase in recombination energy E, which consequently leads to a decrease in the emission wavelength when compared to that of PersL.In a similar situation, photoexcitation is responsible for a wider distribution of donor-acceptor pair distances, which manifests as a broader band shape relative to PersL.Furthermore, our previous research has established that D-A emission is directly proportional to the excitation intensity, where higher excitation power results in a reduced separation distance between donor and acceptor, thereby inversely affecting the recombination energy (Laser Photonics Rev. 2023, 17, 2300132).Such D-A luminescence mirrors the interactions between the activator (typically Cu + ) and the coactivator (such as Cl -or Al 3+ ), as seen in the luminescence dynamics of ZnS-based electroluminescent or PersL materials (J.Electrochem.Soc. 1956, 103, 342;J. Electrochem. Soc. 1953, 100, 72).
We have incorporated a discussion on the characteristics of D-A emission in both PL and PersL into the revised manuscript: "However, the peak wavelength of PL is observed to be slightly shorter compared to that of PersL across the series of Ca(Sr)ZnOS:Cu/Y crystals.This phenomenon is attributed to the significantly reduced average distance between D-A pairs upon photoexcitation, leading to higher emission energy.
Along with the redshifted emission peak, the emission band of PL and PersL are obviously broadened, with the full width at half-maximum (FWHM) varying from 117 to 182 nm for PL and 117 to 170 for PersL (Fig. 3d).The difference in the FWHM between PL and PersL originated from the broader distribution of donor-acceptor (D-A) pair distances in the presence of photoexcitation, which results in a more substantial FWHM for PL when compared to PersL." crystals.e, PersL decay profiles of Ca(Sr)ZnOS:0.1%Cu+ /1%Y 3+ with various Sr contents.The samples were pre-charged using a 365 nm UV lamp (4 W) for 3 min and a short delay of 20 s was allowed before each measurement.
The luminescence properties of Ca(Sr)ZnOS:Y/Cu are notably superior to what is typically expected from Cu 2+ ions with a 3d 9 electron configuration.Should Cu 2+ ions play a role in the luminescence process, one would expect to observe emissions or absorptions in the nearinfrared spectrum, similar to the behaviour of ZnS:Cu 2+ , rather than a visible broadband emission.Regarding the possibility of Cu-related defect luminescence, the spectral analysis of Ca(Sr)ZnOS:Cu samples has been presented.These specimens demonstrate donor-acceptor (D-A) features, with an emission peak observed between 500-550 nm, indicative of donor sites potentially originating from Cu-induced charge-compensating defects.
We have added the following discussion in the revised manuscript: "The luminescence properties of Ca(Sr)ZnOS:Cu/Y significantly exceed the usual expectations for Cu 2+ ions with a 3d 9 electron configuration.If Cu 2+ ions were involved in the luminescence, emissions or absorptions would likely be in the near-infrared range, similar to the behaviour of ZnS:Cu 2+ , instead of the observed visible broadband emission.The luminescence seen in Ca(Sr)ZnOS:Cu/Y is reminiscent of D-A pair emission found in ZnS:Cu -/Al 3+ (or ZnS:Cu + /Cl -), which suggests that the emission does not stem from isolated ionic emission of Cu 2+ or Cu + .This observation supports the idea that the luminescence process in Ca(Sr)ZnOS:Cu/Y involves a more complex interaction than simple ionic emission."2. Following the above comment, the previous publications on luminescent properties on Ca(Sr)ZnOS:Cu or its derivatives, if relevant, should be cited and discussed in this paper.

Response:
We appreciate your suggestion.Tu et al. explored the mechanical quenching of PersL in CaZnOS:Cu + (Light Sci.Appl. 2015, 4, e356;Appl. Phys. Lett. 2014, 105, 011908).They concluded that the luminescence originates from a donor-acceptor (D-A) pair, formed by interactions between nearby trap levels and the Cu 2+ (photoionized Cu + ) energy state.Our previous studies have indicated that the introduction of RE 3+ (where RE represents rare earth elements) modifies the donor-acceptor (D-A) emission spectrum, which can be further finely tuned by Sr 2+ alloying, as detailed in our work.
We have cite related reference (ref. 37-38) and added the following discussion in the revised manuscript: "The study by Tu et al. has unveiled D-A emission characteristics through their research on the mechanical quenching of PersL in CaZnOS:Cu + .Concurrently, our prior investigations have demonstrated that in CaZnOS, copper and rare earth elements (e.g., Y, Gd, Tb, Nd, Er, Ho, Tm, Dy and Pr) are capable of forming donor-acceptor pairs.This interaction facilitates band emissions, which occur alongside the intrinsic lanthanide luminescence." 3. The thermoluminescence spectra in Figure 4e contain abnormal spectral signals.Please verify these results.

Response:
We apologize for any confusion.The TL measurement instrument has been upgraded to a domestic model, denoted as TL/OSL spectrometer (LTTL-3DS).This advanced device is capable of resolving wavelength information, which adds an additional dimension to our understanding of the TL signal.By integrating across wavelengths, we can derive the conventional two-dimensional TL glow curve, which depicts TL intensity as a function of temperature.Moreover, by rigorously controlling the testing conditions (pre-charging wavelength/time, delay time, heating rate), we have determined that the profile of TL glow curves kept nearly unchanged while their peak maximums shift slightly to the lower temperature side (from 359 K to 348 K at a constant heating rate of 1 K/s) as the Sr concentration increases.This finding substantiates our hypothesis that alloying with Sr does not significantly alter the trap distribution; nonetheless, it consistently results in a decrease in the values of trap depth.For additional information, please refer to the detailed explanation in response to Question 4. The result has been incorporated into Fig.4e in the revised manuscript.In addition, the original three-dimensional contour plots of TL spectra have been included in Supplementary Fig. 21.Owing to their consistent emission characteristics, TL also exhibits a spectral profile that can be finely adjusted, similar to that of room temperature PersL.
We have added the following discussion in the revised manuscript: "However, the maximum of the TL glow peak shifts to a lower temperature (from 359 K to 348 K), indicating a consistent decrease in the trap depth.Additionally, the observed shift in the TL emission peak, in correlation to an increase in Sr content, is consistent with the behaviour noted in room-temperature PersL, indicating a similar nature between TL and PersL (Supplementary Fig. 21)."+ /1%Y 3+ (x = 0, 0.1, 0.25, 0.4, 0.55, 0.7, 0.85 and 1).The heating rate # was varied from 0.5 to 2.5 K/s and TL peak maximum Tm was determined from the TL glow curves.k is Boltzmann constant.The linear fitting result is included in each subplot.Response: We apologize for any confusion.The multicolor display using the as-prepared PersL phosphors is achieved by charging a flat-panel PersL film (PersL crystals encapsulated in PDMS matrix) covered by a patterned PET film.As shown in Fig. 5a, the PET film with a predefined graphic acts as a photomask, selectively attenuating the charging light based on the target display shape.After charging, without removing the PET film, a multicolor graphic becomes visible.Thus, the PET film serves as both a mask at the charging stage and a color filter at the display stage.
For the proof-of-concept demonstration, we initially achieved super broadband persistent luminescence (PersL) emission in mixed Ca(Sr)ZnOS:Cu + /Y 3+ crystals (with Ca0.75Sr0.25ZnOS:Cu+ /Y 3+ and Ca0.45Sr0.55ZnOS:Cu+ /Y 3+ at a weight ratio of 2:1).These crystals were then used to fabricate the PersL film for display applications.In Fig. 5h, the rightmost panel is based on Ca(Sr)ZnOS:Cu + /Y 3+ crystals, while the middle panel serves as a comparison and is based on Ca(Sr)S:Eu 2+ -a red PersL phosphor with a narrow emission band.
6.I would like to suggest the authors to move Supplementary Figure 18 into the main figures because this is important to understand the charge transfer process in the studied materials.

Response:
We are grateful for your valuable suggestion.We have incorporated the schematic illustration of PersL mechanism into Fig.4g of the revised manuscript.
Accordingly, we have added the following discussion in the revised manuscript: "The continuously decreasing trap depth suggests a more significant reduction in the band edge with respect to trap/donor levels.As a result, a global trap model is utilized, conceptualizing trap depth as the relative energy of VS levels to the conduction band (Fig. 4g)." 1.The authors report on the luminescence properties of a particular phosphor based on (Ca,Sr)ZnOS:Cu,Y, where tuning of the luminescence (and of the persistent luminescence) can be obtained by changing the Ca-Sr composition.Also a study on the partial substitution of S by Se is mentioned, but this is less relevant, as an increase in the Se concentration quickly reduces the trapping capacity of the phosphor.

Response:
We greatly appreciate your insightful feedback, and we wish to emphasize the core achievement of our research.As detailed in the manuscript, we have developed a method for continuously tuning persistent luminescence (PersL) within a single material system, for the first time.To more effectively showcase our material's superiority compared to those documented in existing literature, we have included a summary table (Supplementary Table 1) that encapsulates earlier attempts at PersL tuning.The investigation revealed that the conventional approach to modulating PersL typically involves combining various emission centers-such as triplet states, guest chromophores, and inorganic activators-within one host matrix.While these methods do result in distinct emission wavelengths, they inevitably leave spectral gaps in specific wavelength ranges, restricting the full spectrum emission of PersL.As outlined in the manuscript, we discovered that the PersL emission peak can be finely adjusted in a near-linear fashion across an extensive wavelength span (500-635 nm), with a remarkable precision of approximately 5 nm.This discovery permits the on-demand tailoring of PersL color (spectrum) spanning a wide spectral range of over 100 nm, which is unparalleled in any other material system.
The findings related to Ca0.45Sr0.55ZnOS(Se):Cu/Yare presented with the intent of demonstrating the predominance of sulfur on PersL.It has been observed that PersL diminishes progressively as the sulfur content decreases (more S is substituted by Se).Although this aspect is not the main focus, it serves as an additional insight to comprehend the origin of PersL within Ca(Sr)ZnOS:Cu/Y systems.We hope you concur.2. One of the reasons for this study is mentioned in the introduction: PersL emitters of various emission colors usually exhibit unbalanced brightness and efficiency.Unfortunately, this is also the case in the present solution: changing the Ca-Sr ratio also changes the trapping behaviour (as witnessed in the TL glow curves and in the intensity of the decay curves (Fig. 3).In that sense, the present phosphor system is not a real solution, where spectral tuning can be achieved independently of the trapping behaviour.

Supplementary
Response: Thank you for raising this point.Following an accurate control of the TL measurement conditions (charging wavelength/time, delay time, heating rate etc.), we find that altering the Ca/Sr ratio has a minor impact on the trapping behaviour of the PersL phosphor (Fig. 4e).The TL spectral profile remains largely unchanged, although its peak maximum slightly shifts to a lower temperature with the addition of Sr.On one side, As previously addressed, the primary focus of this work is the continuous tuning of PersL wavelength in a single material system.On another, "brightness" is a concept determined by human visual perception, which is inherently influenced by wavelength.Even in the well-established CsPbX3 (X=Cl, Br, I) system, achieving emission with equal brightness across a spectrum of colors remains a challenge.To more accurately reflect the storage capability of our PersL phosphors, we have employed an integrated TL intensity as a figure of merit.The result is depicted in Fig. R2, where the coefficient of variation of integrated TL intensity across eight samples is 24.95%, endorsing the achievement of "wavelength-tunable PersL with comparable emission intensity".
To the best of our knowledge, no prior research has shown the ability to finely tune PersL across a range of wavelengths while maintaining uniform trapping behaviours and comparable storage capacities.We hope you concur.3. Of course, the study by itself contains interesting results, but I am not convinced that the proposed material system will have a high impact in the field, given that the aforementioned problem is not really solved.In the application part, a 'super broadband PersL' is created by mixing two Ca-Sr compositions.Doesn't this create the same issue, regarding color changes during the decay?
Response: We appreciate your recognition of the strengths of our study.As outlined in Supplementary Table 1, color tuning on PersL is a constant subject of research.Therefore, we believe that achieving continuous tuning of PersL with high wavelength accuracy in a single material system will be of significant interest to the field.
Furthermore, we are confident that color variation will not be an issue for the proposed demonstration.For confirmation, we recorded the PersL spectrum of the two-component mixture to check whether it will change over an extended period, with the findings detailed in Fig. R3.The broadband PersL spectrum exhibits negligible shifts over time, effectively mitigating any worries regarding color alteration throughout its operation.We hope you concur.4. Furthermore, in the abstract it is claimed that the high initial brightness is up to 5 cd/m², although I could not find data supporting this claim.All the data for the decay curves are shown in arbitrary units, except for supplementary Figure 9, but there the initial intensity is lower than 5cd/m².Furthermore, it is mentioned that it was measured with a (scanning) spectrometer, calibrated with a luminance meter.More details should definitely be provided on the calibration method, as the luminance (measured in cd/m²) depends on the shape of the spectrum.Hence, a calibration of the FLS980 spectrometer to yield cd/m² values for widely different emission colors is not straightforward.The advise would be to measure the phosphors directly, after excitation, with the luminance meter, and then compare them.Preferably, also standard excitation conditions (Xe lamp and D65 lamp) should be used to give a fair estimate of the obtainable brightness.
Response: We regret any misunderstanding caused.The initial brightness refers to the luminance value (captured by a CHROMA METER CS-200 luminance meter) 5 seconds after turning off the charging light (a 4W 365 nm lamp for 3 minutes).The luminance values measured at different delay moments for the Ca(Sr)ZnOS:Cu/Y samples are detailed in Supplementary Table 3.However, owing to the luminance meter's detection limit being 0.01 cd/m 2 -significantly higher than the threshold value 0.032 mcd/m 2 -it was not possible to accurately determine the PersL duration of our materials.To rectify this, it is essential to calibrate the PersL decay curves using the recorded luminance values.The calibration assumed that the luminance follows the same trend as the PersL decay curves, and the charging light source and delay time are kept the same for both PersL decay and luminance measurements.
The decay curves of PersL were recorded using the FLS980 spectrometer, set to kinetic scan mode, which tracks the intensity decay at a specific wavelength.In a detailed measurement process, the sample was initially charged with a 365 nm handheld lamp (4W) for 3 minutes.Subsequently, the sample was placed inside the spectrometer's chamber.A fixed delay of 20 seconds was allowed between the end of the charging and the commencement of the measurement.For luminance calibration, the luminance value at 20 s after the ceasing of optical charging were utilized.This corresponds to a value of 1.14 cd/m 2 for Ca0.6Sr0.4ZnOS:0.1%Cu+ /1%Y 3+ as indicated in Supplementary Fig. 10 and documented in Supplementary Table 3 in the revised Supplementary Information.
In accordance with your recommendation, we employed a D65 lamp as a charging light source, which has been proven to be able to excite CaZnOS:Cu/Y crystals.However, due to the limited spectral overlap between the emission of the lamp and the PersL excitation of our PersL material (Supplementary Fig. 8), the luminance values were lower than those previously measured.The results are compiled into the Supplementary Table 4.
Furthermore, our materials have demonstrated the capability to emit higher brightness when subjected to a high-power excitation source.In our experiments, we charged the Ca(Sr)ZnOS:Cu/Y crystals using a 10 W 365 nm lamp.The resulting brightness values shown below in Table R1.Notably, a peak luminance of 12.11 cd/m 2 was observed at 5 seconds after the excitation source was turned off.We have added the following statement in the revised manuscript: "Particularly, these materials are distinguished by their remarkable ability to be excited by a standard D65 lamp (Supplementary Fig. 8 and Supplementary Table 4), considerably broadening their applicability in lighting, displays and safety indications." We have added additional information regarding the PersL decay curves and luminance calibration in the revised Supplementary Information: "The decay curves of PersL were recorded using the FLS980 spectrometer, set to kinetic scan mode, which tracks the intensity decay at a specific wavelength.In a detailed measurement process, the sample was initially charged with a 365 nm handheld lamp (4W) for 3 minutes.Subsequently, the sample was placed inside the spectrometer's chamber.A fixed delay of 20 seconds was allowed between the cessation of the charging and the commencement of the measurement.For luminance calibration, the luminance value at 20 s after ceasing the optical charging was utilized.The calibration assumed that the luminance follows the same trend as the PersL decay curves, and the charging light source and delay time are kept the same for both PersL decay and luminance measurements." Fig. R1.a) Photoluminescence decay and b) transient emission spectra of the representative Ca0.45Sr0.55ZnOS:Cu/Ysample.

Fig. 4f .
Fig. 4f.Calculated trap depth as a function of Sr content in the series of Ca(Sr)ZnOS:Cu/Y samples. 5.It is not clear enough how to obtain the results of Figure 5h.What is the meaning of patterned filter?Did the author use the synthesized PersL phosphors as lighting sources in the two right panels?

Fig. R2 .
Fig. R2.Integrated TL intensity as function of Sr content.

Fig. R3 .
Fig. R3.The stability of the PersL spectra for the two-component mixture.

Table 1 .
Brief summary of literature work on PersL spectrum tuning.

Table 1 .
Brief summary of literature work on PersL spectrum tuning.

Table R1 .
The brightness of the newly developed Ca(Sr)ZnOS PersL materials (Charged by a 10 W 365 nm lamp).